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The application of x-ray absorption spectroscopy methods to both materials and life sciences is well appreciated. However, the power of extended x-ray absorption fine structure (EXAFS) spectroscopy as a quantitative structural technique has largely been limited by its application to the microscopically homogeneous systems, in which the local environment around each absorbing atom in the sample is the same. The growing interest in time-resolved EXAFS studies of systems in physics, chemistry, biology, and materials science has reintroduced the requirement for an analytical tool to probe heterogeneous mixtures in situ. While long being recognized as a premiere technique for this role, EXAFS studies of mixtures have been particularly difficult due to the strong model dependence and correlations between parameters in the fit. To circumvent these drawbacks, we introduce two new techniques in EXAFS analysis: the principal component analysis and the residual phase analysis. Using a test case of a heterogeneous mi...

X-ray specular reflectivity has been used to characterize the structure of silicon--silicon-oxide surfaces coated with chemisorbed hydrocarbon monolayer films (alkylsiloxanes). Using synchrotron radiation the reflectivity was followed over 9 orders of magnitude, from grazing incidence to an incident angle of \ensuremath{\varphi}\ensuremath{\approxeq}6.5\ifmmode^\circ\else\textdegree\fi{}, or q=(4\ensuremath{\pi}/\ensuremath{\lambda})sin(\ensuremath{\varphi})=0.8 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, allowing a spatial resolution of features approximately \ensuremath{\pi}/0.8\ensuremath{\approxeq}4.0 A\r{} along the surface normal. Analysis was performed by fitting the data to reflectivities calculated from models of the surface electron density and by calculating Patterson functions directly from the data. Although the measured reflectivities could be equally well described by different sets of model parameters, the electron densities calculated from these different parameters were remarkably alike. Inspection of the electron densities allowed identification of a layer of ${\mathrm{SiO}}_{2}$ (\ensuremath{\approxeq}17-A\r{} thick), a layer of head-group region where the alkylsiloxane adsorbs to the ${\mathrm{SiO}}_{2}$, and the hydrocarbon layer. Fitting the data also required that the various interfaces have different widths. The fact that the same local hydrocarbon density of 0.85 g/${\mathrm{cm}}^{3}$ was observed for both fully formed and partially formed monolayers with alkane chains of varying length excluded a model in which the partially formed monolayer was made up of separated islands of well-formed monolayers. Measurements before and after chemical reaction of a monolayer in which the alkyl chain was terminated by an olefinic group demonstrated the ability to use x-ray reflectivity to characterize chemical changes. The effects of radiation damage on these types of measurements are described.

The use of principal component (factor) analysis for the interpretation of EXAFS spectra is described. The components derived from EXAFS spectra share mathematical properties with the original spectra. As a result, the abstract components can be analyzed using standard EXAFS methodology to yield bond distances and other coordination parameters. The number of components that must be analyzed is usually less than the number of original spectra. The method is demonstrated using a series of spectra from aqueous solutions of uranyl ions.

X-ray absorption spectroscopy has often been used to examine the local environment around an absorbing cation in both the solid and the liquid state. In order to examine the properties of a transition-metal ion under conditions in which the degree of solvation can be regulated, the authors have placed cupric ions within a smectite clay. Smectite clays consist of negatively charged sheets of aluminosilicates which are separated by an interlayer whose thickness changes upon absorption of solvent. In the native clay used for these experiments, bentonite, the calcium was replaced with copper by stirring the clay in a 0.1 M aqueous solution of CuCl[sub 2]. For the experiments described here, Cu K-edge spectra were acquired at the National Synchrotron Light Source and Stanford Synchrotron Radiation Laboratory in both transmission and fluorescence modes. Four samples of Cu-bentonite powder, dried from water, methanol, ethanol, and ethylene glycol, were examined, as well as slurries of these samples in their respective solvents. The normalized X-ray absorption near edge (XANES) spectra are obtained for copper ion in the Cu-bentonite dried from methanol and for the Cu-clay as a slurry in the same solvent. The spectrum of the dry material clearly shows the electronic 1s-4p transitionmore » as a resolved shoulder on the absorption edge at 8986 eV. This transition, which is characteristic of a copper ion with square planar coordination, has broadened and almost disappeared in the slurry. The observed change suggests that as the solvent penetrates into the clay, some of the solvent molecules enter the coordination sphere of the copper(II) ion. The addition of two axial oxygen ligands shifts the coordination geometry of the Cu(II) from a square planar configuration to a distorted octahedron. There is an obvious change in the EXAFS data between the dry clay and the slurry which cannot be readily discerned by examination of the radial distributions alone. 23 refs., 2 figs., 1 tab.« less

The thicknesses of C10-C18 alkylsiloxane monolayers on silicon-silicon dioxide substrates have been measured with ellipsometry and low-angle X-ray reflection. Although, for any given sample, thicknesses measured by the two methods agree to within experimental error, ellipsometric measurements are systematically larger by approximately 2 {angstrom}. This difference may result from variations in the sensitivity of the two techniques to the structure of the interface between silicon dioxide and the alkylsiloxane monolayer. The x-ray reflectivity measurements provide evidence that these organic monolayers do not build up as island structures and demonstrate that the approximate area projected by each alkyl group in the plane of the monolayer is {approximately} 21 {plus minus}3 {angstrom}{sup 2}. Preliminary studies indicate that this technique can be used to follow the changes in the structure of a monolayer which result from chemical transformations. The influence of damage that is induced by x-ray radiation on these measurements is discussed.